Sexual transmission of HIV-1, the major mode of HIV-1 infection world-wide, occurs across mucosal surfaces. In the face of an expanding epidemic there is an urgent need for effective strategies to block mucosal spread of this infection. However, success of any preventative strategy may be critically dependent upon detailed knowledge of the mechanisms of transmission. While much has been learnt about vaginal and/or rectal transmission, relatively little is known about the mechanisms of HIV transmission to male genital tissue. Indeed, factors governing such transmission and the events between exposure of male genital mucosal surfaces and establishment of clinical infection are still poorly understood. The number and frequency of target cells, the relative susceptibility of different tissue sites (foreskin, glans and urethra) and the mechanisms of trans-epithelial transmission have not been investigated in a systematic fashion. This project aims to test the hypothesis that tissue specific determinants govern relative susceptibility to HIV-1 infection and replication in male genital tissue. In particular, the relative frequency of potential target cells, and receptor expression, will be correlated to ex vivo susceptibility of male genital tissue explants (foreskin, glans and urethra) to HIV-1 infection (cell free and cell associated, using a range of isolates). Male genital tissue will be obtained, with consent, from gender reassignment and other surgical procedures, and optimal culture conditions will be determined using techniques developed for other mucosal explant models (cervical/rectal) already established in the applicant's laboratory. To determine potential mechanisms for any differential susceptibility to HIV infection, the principal target cells for infection, and/or uptake of virus by migratory cells within the different tissue sites will be defined. The use of specific receptor/co-receptors in such mechanisms will be confirmed using a range of available antagonists and blocking Mabs. These studies may provide a major contribution to our understanding of the determinants of HIV-1 infection of male genital tissue, which in turn could be key to successful targeting of strategies and topical microbicides designed to block the sexual spread of HIV infection. The proposed "R21 grant" builds on the applicant's proven track record in investigation of HIV replication in cervical and rectal tissue models, and will bring added value to a number of NIH funded microbicide related programme project grants already underway in the applicant's laboratory.